Preparation and Processing of Monodisperse Colloidal Silica-Cadmium sulfide nanocomposites
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crystalline colloidal arrays (CCA) where the colloidal spheres are prepared from nonlinear materials. 8 High incident light intensities alter the sphere refractive indices such that the array diffraction properties change. The nonlinear CCA would act as an optical limiter or switch. This application requires a material with a high nonlinear coefficient. CdS quantum dots may have among the largest optical nonlinearities known. Thus, we have developed synthetic methods to prepare CdS quantum dot inclusions in much larger monodisperse colloidal spheres. This article describes the fabrication of a new nanocomposite composed of CdS quantum dots (ca 25 A) uniformly dispersed in monodisperse silica spheres (40-150 nm). Furthermore, this method can be applied to fabricate nanocomposites where the CdS forms a core within each sphere, or where the CdS forms patches on the surface, or where it forms multidecker shells of silica, CdS, silica, etc.
INTRODUCTION Development of methods to synthesize monodisperse particles is important for the advancement of numerous areas of science and technology 1 . The development of new methods which give well defined but complex morphologies will present new opportunities in materials science. In this paper we describe a novel methodology to prepare monodisperse composite particles in the 50 nm to -300 nm size range. Our major interest is to use these spherical particles in for colloidal self-assembly processes 28 preparing crystalline colloidal arrays " useful for novel optical devices. These nanoscale periodic materials are useful for fabricating optical devices because the spherical particles array themselves at the lattice sites of body centered cubic or face centered cubic arrays; these arrays efficiently diffract light in the visible and near IR spectral regions. We are developing optically nonlinear
875 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
MATERIALS AND PROCEDURES
hydrolysis and condensation of TEOS within Igepal reverse micelles. The particle size and size distribution are sensitive to the microemulsion formulation, i.e. the ratio of water to surfactant concentration, w, and the ratio of water to alkoxide concentration, h. The polydispersity depends strongly on the alcohol concentration because of the sensitivity of the microemulsion phase diagram to the alcohol content. Increasing the alcohol concentration generally increases the polydispersity. This therefore limits the maximum concentration of TEOS which can be used for good monodispersity since the TEOS hydrolysis and condensation releases ethanol. Nevertheless, we find that higher concentrations of TEOS can be utilized in microemulsions containing Triton N-101 because this microemulsion is less sensitive to alcohol content. In this case the presence of the co-surfactant hexanol improves the particle size distribution. The reaction kinetics were examined by monitoring the particle size evolution using TEM. Figure 1 shows the effects of w and h on particle growth in Igepal CO-520 microemulsion
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